The field covers a ported cylinder for an opposed-piston engine. More particularly, the field relates to impingement cooling of a ported cylinder in a two-stroke opposed-piston engine.
In a two-stroke, opposed-piston engine, two pistons are disposed in opposition in the bore of an elongated cylinder. Exhaust and intake ports are provided through the cylinder sidewall near respective ends of the cylinder. When the engine operates the pistons slide toward and away from each other in the cylinder bore. As the pistons slide together, air is compressed between their end faces and combustion occurs when fuel is injected into the compressed air. The piston end faces contain combustion in a relatively narrow cylindrical space in the cylinder bore, whose side is defined by a circumferential portion of the cylinder sidewall that is substantially centered between the exhaust and intake ports. This circumferential portion is referred to as the central band of the cylinder. As the piston's slide away from the central band in response to combustion, they open the exhaust and intake ports to enable uniflow scavenging wherein pressurized air flowing into the cylinder bore via the intake port forces combustion products out of the bore through the exhaust port.
A cooling system construction for such a two-stroke opposed-piston engine is substantially different from that of a four-stroke engine. In an opposed-piston engine, combustion concentrates the thermal load at the central band, and the unidirectional flow of air during scavenging results in a non-symmetrical distribution of heat from the central band toward the ends of the cylinder. That is to say, while the central band is the hottest portion of the cylinder, the exhaust end of the cylinder is hotter than the intake end. This asymmetric thermal loading causes longitudinal and circumferential distortions of the cylinder. Distortions of the cylinder lead to increased friction between the pistons and cylinder bore, scuffing of the bore, and reduced durability of the engine.
The high concentration of heat in the central band poses another threat to engine lifetime. As combustion occurs, the opposed pistons (not shown) pass through top dead center (TDC) locations. After the pistons TDC, they reverse direction and begin to move away from each other in response to the pressure of combustion. As reversal begins, combustion causes a sudden rise in pressure in the central band that seats the rings of each piston firmly against a bore surface zone (“the top ring reversal zone”) that overlaps the central band. The spike in friction between the rings and the bore can cause increased wear of the bore surface. Thus, it is important to engine durability that the lubricating oil film in the top ring reversal zone be preserved in the face of the thermal load borne by the central band.
A simple cooling construction for a two-stroke, opposed-piston engine includes a jacket within which liquid coolant flows along the cylinder sidewall in an axial direction from an inlet near the intake port to an outlet near the exhaust port. For example, in the cylinder liner cooling construction described in U.S. Pat. No. 6,182,619, liquid coolant flows over the external surfaces of a cylinder housing, in a direction from the intake end to the exhaust end. However, this construction yields uneven cooling both longitudinally and circumferentially about the cylinder housing.
Improved thermal response has been achieved in a ported cylinder for an opposed-piston engine by introducing the coolant near the central band and providing means to transport the coolant from the central band toward either end of the cylinder. See FIG. 3E of US 20100212613 wherein coolant flows into a circumferential groove on the exterior surface of a portion of the cylinder sidewall in the central band, and through longitudinal grooves on either side of, and in liquid communication with, the central groove toward the intake and exhaust ends of the cylinder structure. FIGS. 11A and 11B of US 2009/0293820 show a cylinder cooling construction including three groups of grooves in the sidewall of a cylinder liner. A group of grooves runs in the direction of the central circumference of the cylinder liner. Separate groups of grooves extend longitudinally from either side of the central group of grooves, toward respective ports. A sleeve disposed over the central band provides separate input ports for each group of grooves.
In the central band of both of these constructions, there are circumferential grooves and openings for injectors that significantly weaken the cylinder structural integrity at the central band and raise cylinder reliability and durability issues. Further, the heat transfer coefficient of coolant surface flow is limited by coolant flow velocity and local geometry, so the cooling capacity in the central band is limited in its effectiveness. This can cause the temperature of the cylinder structure at the central band to exceed the design limits of the cylinder material, leading to excessive liner distortion that causes exhaust blow-by, stress concentration on the cylinder, and excessive ring and cylinder bore wear.
These problems are avoided by a cylinder cooling system for a two-stroke opposed-piston engine that combines mechanical reinforcement of the central band with impingement cooling of the central band, flow cooling of portions of the sidewall between the central band and the ports, and reservoir cooling of portions of the sidewall in the vicinity of the ports.
One objective of such a cooling system is to reduce the thermal variance in the longitudinal and circumferential dimensions of the cylinder in order to maintain its linearity and circularity. Ideally, this objective is achieved by cooling the cylinder in an asymmetric manner that is the inverse of the asymmetrical manner in which it is thermally loaded during engine operation.
It is an objective to provide such cooling while maintaining the structural integrity of the cylinder by strengthening the central band area. In one preferred instance, the cylinder's structural integrity is maintained by elimination of circumferentially-directed grooves in the central band.
Another objective is to limit the temperature of the cylinder in the ring reversal zone so as to prevent or mitigate loss of viscosity and burn-off of the lubricating oil film. This objective is achieved by direction of impingement jets of liquid against the sidewall in or in the vicinity of the central band where piston rings encounter the highest levels of heat and bore distortion.
It is an objective to provide such reinforcement and cooling of the central band while reducing and equalizing the wall temperature and distortion along the whole stroke length of the cylinder as well as around the bore circumference, so as to increase cylinder liner and piston ring reliability and durability.